Practical application of a fuel cell that supplies hydrogen as a fuel gas to a fuel electrode, and that supplies air as an oxidant gas to an oxidant electrode, and that generates electricity through an electrochemical reaction between hydrogen and oxygen in the air while producing water on an oxidant electrode is now being considered.
In such a fuel cell, if at the time of start of operation, the pressure of hydrogen supplied to the fuel electrode and the pressure of air supplied to the oxidant electrode are about equal to the respective pressures occurring during ordinary operation, it sometimes happens that hydrogen gas and air are unevenly distributed in the fuel electrode and the oxidant electrode, respectively, and the electrodes are degraded by electrochemical reaction caused by the uneven distribution of these gases. Japanese Patent Application Publication No. 2007-26891 (JP-A-2007-26891) discloses a method of preventing the degradation of the electrodes of a fuel cell by causing the pressures of hydrogen and air supplied to the fuel electrode and the oxidant electrode, respectively, at the time of start of operation of the fuel cell to be higher than the ordinary supplied pressures of these gases.
However, if hydrogen gas and air are supplied at high pressure to a fuel cell when the fuel cell starts operation, it sometimes happen that the rate of rise of the voltage of the fuel cell becomes large so that the voltage of the fuel cell overshoots its upper-limit voltage. In conjunction with this problem, Japanese Patent Application Publication No. 2007-26891 (JP-A-2007-26891) discloses a method in which when hydrogen gas and air are supplied, at the time of starting a fuel cell, at pressures that are higher than their pressures given during ordinary power generation, output electric power is extracted from the fuel cell, and is put out to a vehicle driving motor, resistors, etc., provided that the voltage of the fuel cell reaches a predetermined voltage that is lower than the upper-limit voltage.
In a fuel cell system mounted in an electric vehicle, an FC relay is provided for turning on and off the connection between the fuel cell and an electric motor. Using the FC relay, the fuel cell is cut off from a load system when the fuel cell is stopped, and the fuel cell is connected to the load system when the fuel cell starts operation. However, there is possibility of the FC relay being welded or damaged if large current flows through the FC relay when the FC relay is closed to connect the fuel cell and the load system.
A countermeasure for the foregoing possibility or risk is to use a method in which when the fuel cell starts, the voltage of the fuel cell is temporarily raised to an open-circuit voltage to attain a state where current does not flow out of the fuel cell, and then the FC relay is connected, and after that, the voltage is lowered so that electric output is supplied from the fuel cell. This is because the fuel cell has a characteristic that if the voltage rises to the open-circuit voltage, the output current becomes zero. However, if the voltage of the fuel cell is raised to the open-circuit voltage, the high voltage can affect the durability of the fuel cell.
Besides, in some cases, at the time of starting the fuel cell, the FC relay is connected while the voltage of the fuel cell is low, so that large current does not flow when the FC relay is connected. In such cases, however, the difference in voltage between the electrical system and the fuel cell is likely to be large, giving rise to the possibility that the transitional current occurring at the time of connecting the FC relay may cause an electrical leakage detector to falsely detect electrical leakage, and therefore the control portion may stop the fuel cell system.